This is the description of the minimal interface between L1 and higher layers (L23)neccessary to establish a dedicated channel with the cell and perform procedures likeLOCATION UPDATE or AUTHENTICAITON.

Instead of a protocol with a fixed header (including length of payload), we chose a minimalisticsubset of HDLC. This has the advantage that it provides multiplexing between up to 255 differentchannels, and it is auto-synchronzing, i.e. when data is lost, the next frame start will still berecognized by the other siede.

The UART in the Calypso DBB can only run up to 115200bps with standards-compliant baud rates.Higher baud rates are possible (up to 812500bps), but only as non-standard rates. While thereare serial chips like FTDI's chip that can support this, regular serial ports and popularPL2303 based cables don't support it.

Therfore, we have to assume that the RS232 link runs at 115200bps.

115200bps means roughly 11,520 characters per second

at a TDMA frame interval of 4.615ms, this means 53 characters per TDMA frame

however, since four TDMA frames are merged into one L1 payload, we have about 212 bytes of RS232 bandwidth for each L1 payload that we receive (260 bits == 32.5 bytes)

This means that regular L1A operation on a single dedicated timeslot is very well possibleeven with lots of overhead for protocol layers, anciliary data as well as a bit of debugoutput.

But it also means that we can not print much debug info from L1 during every TDMA irq.

Also, actual L1A messaging should have a higher priority than serial console messages.It would be great to have a strict priority between those two in the serial driver layerof the phone software.